1. Field of the Invention
This invention relates to a metal and graphite fiber composite produced by molten copper/copper alloy metal infiltration of graphite fibers, and more particularly, to a novel composite having a copper/copper alloy matrix with a reinforcement of graphite fibers for improved strength and heat transfer at a decreased weight.
2. The Prior Art
Numerous applications require a material of construction having the characteristics of high thermal conductivity, high strength, and relatively low weight. For example one application for a material of construction having these characteristics is a fuel handling system for certain types of advanced aircraft engines wherein the fuel is heated rapidly immediately prior to its entering the combustion chamber. The low weight requirement is, of course, self evident from its application. High strength is necessary when dealing with such exotic fuels as high-pressure hydrogen while high thermal conductivity is necessary to assure adequate heat transfer into the hydrogen prior to its introduction into the combustion chamber.
Additionally, since the invention of gun powder many centuries ago, numerous advances have been made not only in the types of propellants for use in a gun but also in the gun itself. These advances have included high velocity/pressure propellants which have required corresponding increases in the strength of the gun barrel to withstand the extreme pressures generated by these advanced propellants. Customarily, improvements in gun barrel strength are accomplished by improvements in the type and total mass of the steel or other metal alloys from which the gun barrel is fabricated. However, this increase in the mass of these advanced gun barrels increases the weight to unacceptable limits for certain applications. Certain advanced propellants have also been known to cause melting of the surface of the bore surface in the absence of adequate heat transfer even with the firing of a single round. Advanced propellants along with rapid firing cycles generate excessive amounts of thermal energy which must be dissipated adequately in order to prevent thermal degradation of the gun barrel. Heat buildup can cause premature initiation of the propellant in the next round brought into the firing chamber during the rapid firing sequences. Accordingly, the low thermal conductivity of conventional as well as advanced steel alloys limits the duration of the firing burst.
A material to provide these desired strength and heat transfer capabilities would be a composite material prepared from a graphite fiber reinforced copper/copper alloy matrix. The graphite fibers are prepared in the desired configuration and then infiltrated with molten copper/copper alloy. Graphite fibers have strength levels similar to or greater than metal fibers although they have a density significantly less than correspondingly-sized metal fibers so that the specific strength of graphite fibers on a weight-to-weight basis is several times that of metal fibers. Graphite fibers also have a higher modulus of elasticity which will result in stiffer composite structures. Graphite fibers also have desirable high temperature properties superior to those of other reinforcing fibers. The coefficient of thermal expansion is lower in graphite fibers so that the composite is more stable dimensionally when subjected to thermal cycling. Pitch-based graphite fibers have a very high thermal conductivity along the axis of the fiber thereby providing a composite having even superior thermal properties. Additionally, the high conductivity of the copper matrix provides excellent thermal conductivity transverse to the graphite fibers.
Historically, the production of graphite fiber reinforced metal composites required the infiltration of the fibers with the molten metal under high pressure. This process is referred to as "squeeze casting" or "pressure casting". The graphite fiber in the resultant composite was not actually wetted by the metal matrix in many cases, so that any bonding between the fiber and the metal was strictly a physical bonding and not a chemical bonding of metal to fiber. Although conceptually simple, pressure casting is difficult to implement since it requires handling molten metal under high pressure. Another problem with this process is that adjacent, closely packed graphite fibers tend to form a capillary that inhibits infiltration by the molten metal. These resulting voids between the fibers degrade the transverse strength of the matrix. High pressures are especially required to infiltrate thick sections of composite and some of the graphite fibers in the prestructure are displaced during the high pressure infiltration.
Further, since the graphite fibers are not actually wetted by this pressure consolidation processes, only the mechanical forces imposed on the metal matrix join the graphite fibers together in the composite. Inherently, these "bonding" forces are weak because of the lack of atom-to-atom bonding between the metal and the fiber.
An alternative to infiltration of graphite fibers with high pressure molten metal is to consolidate graphite fibers plated with metal either by hot pressing or hot isostatic pressing. Large parts can be formed only with difficulty using this approach. For example, one possible consolidation cycle for copper plated graphite fibers is 1000 psi pressure at 750.degree. C. for 20 minutes. Attempts to produce composite tubes by the applications of external pressure can cause the graphite fibers forming the tube to be compressed laterally and buckle, lending to poor hoop strength properties.
Perhaps one of the more significant problems with copper/copper alloy composites reinforced with graphite fibers is that the graphite fibers dewet from the copper upon exposure to high temperature. This causes the formation of voids in and the swelling of the composite part when exposed to continuous high temperatures or thermal cycling. See, Hutto et al, "Development of Copper-Graphite Composites from Metal Coated Carbon Fibers," 31st International SAMPE Symposium 1145-1153, Apr. 7-10, 1986.
An alternative technology to high pressure consolidation is infiltration of the reinforcing graphite fibers with molten metal at lower pressures. Under these conditions it is necessary to wet the surface of the graphite fibers with the molten metal. If the graphite fibers are wetted by the molten metal, capillary action between adjacent graphite fibers will draw the molten metal into the fibers. The technical background for producing metal matrix composites by molten metal infiltration has been reviewed by Delannay et al., "The Wetting of Carbon by Copper and Copper Alloys," Journal of Material Sciences 149-155 (1987).
The advantage of wetting the graphite fibers with the molten metal is that the metal as a liquid infiltrate creates a chemical bonding between the metal and the graphite fibers (as opposed to mere mechanical bonding as from pressure casting) thereby providing the fiber/metal composite with superior physical properties. Complete infiltration of the graphite fibers with molten metal requires that the graphite fibers must be wetted by the molten metal or metal alloy. However, most metals and metal alloys which will wet the graphite fibers are metals which react with carbon or the graphite form of carbon to form stable metal carbides. Thus, in the process of wetting the graphite fibers, the carbide-forming elements in the infiltrating metal or metal alloy may react in an uncontrolled manner with the graphite fibers, degrading the properties of the graphite fibers. This problem has been addressed by a separate patent of which I am a co-inventor (U.S. Pat. No. 5,244,748 issued 14 Sep. 1993) wherein a barrier layer was interposed between the wetting layer and the graphite fiber, the barrier layer being designed to protect the graphite fiber from attack by the wetting layer.
The production of composite structures of graphite fibers in a copper matrix constitutes a significant problem since pure graphite fibers are not wetted by pure molten copper. At least two approaches have been suggested in the prior art for improving wettability. One approach is to coat the surface of the graphite fiber with a chemical layer that promotes wetting. The other approach is to develop an alloy matrix material that will wet the graphite fiber. Chromium and titanium have been added to the molten copper infiltrate to improve graphite fiber wetting and/or bonding. The concentration of these wetting agents must be maintained at a low level or the processing time at high temperature must be minimized to prevent significant levels of reaction between the graphite fibers and the copper infiltrate thus limiting the strength and utility of the composite. Additionally, the addition of alloying agents may decrease the thermal and electrical conductivity of the metal matrix.
There are many examples in the art of coating graphite fibers either to protect the fibers or to provide for a wetting action on the graphite fiber surface. Various techniques to accomplish this purpose are known in the art and include, for example, using silica as set forth in the foregoing patent to coat graphite fibers in order to subsequently wet the fibers with molten magnesium, aluminum alloys and/or copper/copper alloys. Nickel coatings are readily wetted by molten copper although the molten copper readily dissolves the nickel. Titanium boron coated on graphite fibers will enhance wetting between graphite fibers and a number of molten metals, however these coatings must not be exposed to the air prior to infiltration otherwise the enhanced wetting properties are lost. The requirement that this coating not be exposed to air limits the configuration of composites which can be formed using this coating.
In view of the foregoing, it would be an advancement in the art to provide a technique for producing metal matrix composites by a low pressure liquid infiltration process. Further, it would be an advancement in the art to provide a fiber coating which is stable in the air and which is spontaneously wet by liquid metals, particularly, if the coating allows thick sections of composite to be formed. Such a coating would allow the production of filament wound composites, as well as composites in which the fibers are woven into a structure. It would be also an advancement in the art to use these composites to provide a tube that is lighter in weight while retaining the strength characteristics of a tube fabricated from an alloy steel. It would also be an advancement in the art to provide a high-temperature device such as a gun barrel having a major portion of the alloy steel replaced with a graphite fiber and copper matrix as a gun barrel jacket with a hard, heat resistant liner. It would also be an advancement in the art to provide a method for fabricating a gun barrel wherein a metal alloy or ceramic gun barrel blank or liner is wound with graphite fibers and the graphite fibers wetted with molten copper. Another advancement in the art would be to provide a process that can be used to produce a strong, lighter weight structure having improved strength and heat transfer capabilities.
Such a novel composite and method is disclosed and claimed in this application.